1. Field of the Invention
The present invention relates to an optical record carrier and, in particular, to a track format for use with an optical record carrier having a changing track pitch.
2. Description of the Related Art
The signal to noise ratio of an optical record carrier is dependent upon the density of the pits established on the optical record carrier. As the density of the pits increases, the signal produced by any one pit is increasingly affected by the signals produced by adjacent pits and eventually reaches a point where the signal produced by one pit cannot be distinguished from the signal produced by another pit.
With respect to optical disks, the effect of adjacent pits on a defined pit is typically broken down into a radial component and a tangential component. Providing the sum of the radial component and the tangential component does not exceed, for any point on the recording surface of the optical disk, the threshold at which the signal to noise ratio becomes unacceptable, then the density of the pits and the storage capacity of the disk can be increased.
One method, known as the CAV method, of recording data on an optical disk involves rotating the disk at a constant angular velocity and transferring data at a constant frequency from the write head to the optical disk. One consequence of rotating the disk at a constant angular velocity is that the linear velocity of the disk with respect to the write head increases as the distance of the write head from the center of the disk increases. For example, the velocity of a point on the innermost track with respect to the write head when the write head is established over the innermost track is substantially less than the velocity of a point on the outermost track relative to the write head when the write head is established over the outermost track. Since the rate at which data is transferred from the write head to the disk is constant and the linear velocity of the disk increases as the write head moves away from the center of the disk, the density of pits established on the disk decreases as the write head moves away from the center of the disk. This decrease in the density of pits as the write head moves away from the center of the disk results in a reduction in the tangential component of interference. Consequently, for a given signal to noise ratio, the storage capacity of the optical disk can be maximized by decreasing the track pitch (the radial distance between center lines of adjoining tracks) as the radius of the tracks increases and, as a consequence, increasing the radial interference.
Typically, tracking or following of a particular track on an optical disk is accomplished by generating a tracking signal that is based on the interaction of a laser beam with a structure on the optical disk that defines the track and then using the tracking signal to appropriately alter the position of the read/write head with respect to the optical disk. With reference to FIG. 1A, a known track structure for use in generating a tracking signal is illustrated for two different track pitches. The track structure includes a pair of wobbled pits, a first pit offset to one side of a track and a second pit offset to the other side of the track. Each track is partitioned into a data field where user data is recorded and a servo field where the wobbled pits are recorded. The servo fields and the data fields associated with one track are radially aligned with the servo fields and data fields of adjacent tracks. Moreover, the sequence of wobbled pits associated with one track, when viewed from the boundary between the servo field and the data field, is the same from track to track. Consequently, as the track pitch decreases, the pits begin to run together. Moreover, the amplitude of the tracking signal, which is formed by taking the difference between the signals produced using the first and second pits of a track, decreases. This decrease in amplitude limits the resolution that can be attained with the tracking signal, i.e., the ability to distinguish one track from another track, as the track pitch decreases. The change in amplitude of the tracking signal is illustrated in FIG. 1B for the two track pitches shown in FIG. 1A. The wobbled pits are also offset from the center lines of the tracks by 1/4 of the track pitch. Consequently, as the track pitch decreases, the offset of the wobbled pits with respect to the center lines of the tracks also decreases. This decrease in the offset changes the slope of the tracking signal at its zero-crossing point, the point at which the read/write head is centered over the track. Unfortunately, changes in the slope of the tracking signal at the zero-crossing point also change the gain of the tracking servo loop and, as a consequence, affect the stability of the tracking servo loop. FIG. 1B illustrates the difference in the slopes of the tracking signals at the zero-crossing points for the two different track pitches shown in FIG. 1A. To compensate for the change in slope and stabilize the servo loop, compensating gain circuitry that amplifies the tracking signal based on the track pitch is incorporated into the servo loop.
FIG. 2A shows another known wobbled pit track format at two different track pitches. Characteristic of this wobbled pit track format is that adjacent tracks share a pit. Consequently, the wobbled pits that are radially aligned with respect to one another are always separated by more than a track pitch. This results in a tracking signal, as illustrated in FIG. 2B, that has a substantially constant amplitude over a range of track pitches. The wobbled pits in this track format, however, are always offset from the center lines of the tracks by one half of the track pitch. Consequently, as the track pitch decreases, the offset or distance between the pits and the track center lines also decreases. As with the track format shown in FIG. 1A, this decrease in the offset of the pits with respect to the center lines of the tracks results in a change in the slope of the tracking signal at the zero-crossing point that adversely affects the gain and stability of the tracking servo loop and, as a consequence, requires the incorporation of compensating gain circuitry into the servo loop.
Tracks on optical disks, in addition to including structures for use in generating tracking signals, also typically incorporate reference marks that are established on the track at the same time that user data is recorded on the track and are primarily used to adjust the threshold level that the read electronics uses to determine whether or not a mark of user data has been read from the disk. Adjustment of the threshold level is required to compensate for changes in the signals produced by the marks of user data. These changes are due to variations or fluctuations in the optical media and the like. Incidentally, the reference marks can also be used to adjust the phase of the channel clock, i.e., the clock used to coordinate the transfer of data to and from the optical disk. Presently, known optical disks employ reference marks that are radially aligned from track to track. An example of such a disk is disclosed in U.S. Pat. No. 4,932,017, which issued on Jun. 6, 1990, and is entitled "Method and Apparatus for Reading and Writing Information on a Recording Medium". While the noted patent indicates that the reference marks are to be situated freely so that the decision level or threshold level is not affected by crosstalk caused by adjacent marks, it is believed that the crosstalk referred to is tangential interference and not radial interference. A draw-back associated with using reference marks that are, in the case of an optical disk, radially aligned with one another is that the radial interference between reference marks increases as the track pitch decreases and, as a consequence, the ability to set a reliable threshold detection level diminishes.
Based on the foregoing, there is a need for a track format that can be used to generate a tracking signal that has a substantially constant slope at its zero-crossing point over a range of track pitches without the need for compensating gain circuitry. Moreover, a track format that possesses a constant amplitude over a range of track pitches is also desirable. Additionally, there is a need for track format that can be used to generate a signal that is 90.degree. out of phase with respect to the tracking signal and, as such, can be used to facilitate track capture. There is a further need for track format where the signal produced using the reference marks is less susceptible to radial interference and, as a consequence, produces a more reliable threshold detection level.